This invention relates to a T-joint manufacturing apparatus, and more particularly to an apparatus for manufacturing a T-joint consisting of a tube portion, and a collar portion of which the thickness of a circumferential wall is substantially constant in its circumferential direction.
Known methods of and apparatuses for forming such a collar portion include, for example, the method and apparatus disclosed in Japanese Patent Laid-Open No. 55171/1973.
As shown in FIG. 1, conventional T-joint manufacturing apparatus includes a tube, provided with a pilot hole, clamps 3 for fixing both end portions of the tube 1, a frusto-conical burring punch 4, and an annular heating coil 5 for use in subjecting a circumferential portion, i.e. a portion to be burred of the pilot hole in the tube 1 to high-frequency induction heating by applying thereto a high-frequency current generated by a high-frequency power source (not shown).
In operation, the tube 1 provided with a pilot hole is fixed at its both end portions by using fixing clamps 3, and the burring punch 4 is inserted into the tube 1 and fixed to a rod 6. A high-frequency current is applied to the annular heating coil 5 to subject the portion of the tube 1 to be burred to high-frequency induction heating at a predetermined temperature. The working force 7 is applied to the rod 6 with this working temperature retained, to carry out burring. Consequently, the collar portion 2 is formed to obtain a T-joint. However, such a T-joint manufacturing apparatus has a number of disadvantages. More particularly, as the burring operation progresses, the distance between the portion of the tube 1 being burred and the heating coil 5 varies, so that the working temperature varies causing the working force 7 to fluctuate, and even a slight error in the relationship between the shape of the tube and the shape and position of the collar formed at a portion of the tube 1 causes circumferentially unequal strain to occur in the collar portion 2 during the manufacture of the T-joint.
By virture of the above noted disadvantages, the T-joint manufacturing operation becomes unstable, and the thickness of the wall of the collar portion 2 of the resultant product differs greatly in the circumferential direction thereof. Especially, the thickness of the portions of the T-joint which are in the vicinity of points 2a, 2c in the plane which includes the axes of the tube 1 and collar portion 2, is small, and, conversely, the thickness of the portions of the T-joint which are in the vicinity of the points 2b, 2d in the plane extending at right angles to the above-mentioned plane is large. Accordingly, the strength of the portions of the T-joint which are in the vicinity of the points 2a, 2c is small, and the reliability of the T-joint with respect to its strength is low. Also, cracking 2e occurs in some cases in the vicinity of the points 2a, 2c thereby weakening the T-joint.